This invention relates to an airfoil design for rotating airfoils which has particular application to the blades of a rotor type aircraft (i.e., a helicopter) and aircraft propellers.
Typically, the helicopter forward flight velocity is limited by the increase in airfoil section drag and/or pitching moment that results from the section operational Mach number and lift coefficient. The airfoil section requirements for a helicopter rotor are more complex than those for a fixed wing aircraft. A single design lift coefficient does not exist, because, on a single revolution of the rotor, the airfoil experiences lift coefficients from negative values to maximum lift, and section Mach numbers from low subsonic to transonic value. Thus, the actual operating lift coefficients and Mach numbers for a rotor depend on the specific helicopter design and flight conditions.
In designing airfoil sections for rotor aircraft, the drag divergence Mach number for the airfoil section employed by the aircraft is of considerable importance. Not only do the power requirements increase rapidly when drag divergence is exceeded, but for most airfoils, the section pitching moment will increase substantially at a section Mach number slightly greater than that for drag divergence to result in increases in non-steady blade loads. In many instances over one-third of the rotor disc area operates at Mach numbers above drag divergence.
Prior art airfoil sections are designed to provide low drag and pitching moment; however, drag and pitching moment increases are normally encountered as flight velocity is increased resulting in a poor drag divergent profile. Generally, prior art airfoils encounter large increases in pitching moment coefficient near section Mach numbers of 0.80. A poor drag divergent profile is attributed to one or both of the following factors: (1) local supersonic flow extends behind the airfoil crest at a given lift coefficient to result in an aft facing suction force, and (2) the presence of local supersonic flow can result in shock waves which separate the boundary layer. Both of the above drag increases require additional power to overcome drag, and to propell the aircraft. Thus, drag divergence and pitching moment are particular problems when the aircraft rotor is operating in conditions for high speed flight.
Accordingly, it is an object of the invention to provide an airfoil section which has a near zero pitching moment coefficient about the aerodynamic center at higher section Mach numbers, and a zero pitching moment coefficient about the aerodynamic center at section Mach numbers near 0.80, and which will increase drag divergence to higher section Mach numbers over a broad range of lift coefficients.
Yet another object of the invention is to shape the airfoil sections by regulating the airfoil thickness distribution and camber in a manner to provide improved drag divergence characteristics and near zero pitching moment coefficients.
A further object of the invention is to provide a series or family of airfoils used in conjunction to improve the drag divergence and pitching moment characteristics of a rotor.
Another object of the invention is to provide improved blade stability, reduced blade twist, reduced blade fatigue and/or permit blade construction from a cheaper lighter material.
Still another object of the invention is to provide improved rotor performance to result in increased aircraft stability, increased fuel savings, decreased power requirements and/or permit increases in flight speed.